The present invention relates to an interferometer for measuring a phase difference between a reference beam and an object beam transformed by an optical element, comprising light source means, for generating the object beam and the reference beam, an optical device, for allowing the transformed object beam and the reference beam to interfere in a detection plane, and detection means, for detecting the phase difference between wave fronts of the transformed object beam and the reference beam in the detection plane.
An interferometer of this type is disclosed in U.S. Pat. No. 5,076,695. The known interferometer is intended for measuring the surface accuracy of the spherical surface of an object with a high degree of accuracy, without making use of a comparison surface of an accurately known shape. In the past a comparison surface of this type was placed in the position of the spice to be tested, with the aim of calibrating the interferometer. The known interferometer has first optical means for an essentially monochromatic light beam, originating from a light source, along a first optical axis towards a surface to be tested and directing the reflected beam in the opposite direction along the first axis Furthermore, second optical means are present in order to direct the light beam from the light source as a reference beam along a second axis which crosses the first axis and interference means for directing the reflected beam from the surface to be tested towards the second axis in order to interfere with the reference beam. Detection means are arranged on the second axis in order to measure the interference patterns which are produced by interference of the reflected beam and the reference beam. The Interferometer also comprises point diaphragms, arranged at the point of intersection of the first and the second axis to transform the light beam to the surface to be tested and the reference beam directed towards the detection means into spherical waves. The accuracy that is achieved with this known interferometer is between xcex/100 and xcex/1000.
Increasingly more refined techniques are being used for the production of semiconductor chips, the semiconductor structures becoming ever smaller. An extreme ultraviolet (EUV) lithographic technique will probably be used for the production of structures having a resolution of 0.1 xcexcm, with which technique an optical mask is projected with the aid of a system of mirror as an image reduced in scale onto the substrate on which the structures are produced. The mirrors in this system must be of highly accurate shape in order to obtain the desired effect. The required accuracy of measurement of the shape of the mirrors is approximately 0.1 nm, which at a wavelength of approximately 630 nm (for example an Hexe2x80x94Ne laser) corresponds to xcex/6300. The known interferometer for testing a surface is thus insufficiently accurate for this purpose.
The aim of the present invention is to provide an interferometer which has a very high accuracy compared with the known interferometers.
Said aim is achieved by means of an interferometer of the type defined in the preamble, wherein the optical device comprises:
a first light conductor having an input surface that couples the object beam generated by the light source means into the first light conductor and having a first output surface that generates an object beam having a spherical wave front,
a second light conductor having an input surface that couples the reference beam generated by the light source means into the second light conductor and having a second output surface that generates a reference beam having a spherical wave front, the reference beam being directed onto the detection plane, and
wherein the arrangement of the first light conductor and the optical element is such that the transformed object beam interferes with the reference beam in the detection plane.
Because the optical device of the interferometer does not contain any optical elements between the location where the spherical wave fronts are generated and the detection plane, except for the optical element that transforms the object beam, the (transformed) object beam and the reference beam are not additionally distorted and the interference between the transformed object beam and the reference beam is very clean. Consequently, in principle, a highly accurate measurement of the phase difference between the beam transformed by the optical element and the reference beam in the detection plane is possible, the accuracy being higher than the mum required accuracy (xcex/6300). Furthermore, the interferometer can be used for both reflecting and transmitting optical elements.
One embodiment of the interferometer according to the present invention further comprises processing means for calculating the phase distribution over a specific cross-section of the transformed object beam on the basis of the phase difference between the wave fronts, the positions of the first and second output surfaces of the first and second light conductors, respectively, the position of the detection plane and the position of the specific cross-section.
With this embodiment of the interferometer it is possible to determine the phase difference with respect to the reference beam in a specific cross-section of the transformed object beam. Because both the object beam and the reference beam have spherical wave fronts, this provides information on the transformation of the object beam by the optical element and thus information on the optical characteristics of the optical element.
In an alternative embodiment of the interferometer according to the present invention, the latter comprises processing means for calculating a spatial plane where the phase difference between the wave fronts has a specific shape on the basis of the phase difference between the wave fronts, the positions of the first and second output surfaces of the first and second light conductors, respectively, the position of the detection plane, the position of the spatial plane in general and of a specific point on the spatial plane. Instead of determining the phase difference in a specific cross-section of the transformed object beam, it is possible, with the aid of this embodiment, to determine a spatial plane on which the phase difference has a specific shape. A particular case of this is a shape of the phase difference for which the phase difference is constant. In this case it is then necessary to determine a position of the spatial plane, as well as a point on said spatial plane that defines the constant phase difference.
In a preferred embodiment of the interferometer according to the present invention, the spatial plane is formed by the surface of a reflecting body. Using the interferometer according to this embodiment it is then possible to determine the precise shape of the reflecting body and specifically to do so as the spatial plane in which the phase difference between the object beam and the transformed object beam is equal to the phase difference that is produced by reflection at the reflecting body. In the case of a reflecting body of dielectric material this phase difference is exactly xcfx80. If the reflecting body is not made of dielectric material, the phase rotation is dependent on the angle of incidence of the light beam. The processing means can take this into account. The shape of a reflecting body is also referred to by the term xe2x80x9cform figurexe2x80x9d or the height z as a function of x and y, which is a dimensionless parameter, which is a measure of the shape or asphericity of the reflecting body.
In a preferred embodiment according to the invention, the said positions are determined by a position determination device which is present in the optical device. An output of the position determination device is connected to the processing means. A laser positioning system is used in order to achieve the required accuracy of position determination.
In a preferred embodiment the light source means and detection means of the interferometer are equipped to detect the phase difference between the wave fronts of the reference beam and the transformed object beam with the aid of a combination of heterodyne phase detection means, which determine the phase difference modulo 2xcfx80, and frequency modulation phase detection means for determining the phase difference in multiples of 2xcfx80, the two techniques being employed independently and simultaneously by frequency multiplexing of the light source means.
These two techniques in combination ensure that the optical path difference at a detection point can be determined with sufficient accuracy (the accuracy of heterodyne phase detection techniques is of the order of magnitude of xcex/10,000) and over the desired range of the optical path difference. As a result of the frequency multiplexing of the source means, the two techniques can be performed independently and simultaneously, as a result of which no errors can be introduced by time-shifted measurement of the phase difference modulo 2xcfx80, and the phase difference div 2xcfx80.
Preferably, the interferometer according to the invention is equipped such that the detection means comprise at least one photodetector which is positioned in the detection plane, and that the heterodyne phase detection means comprise a frequency-stabilised light source, a first frequency shift device, connected to the output of the frequency-stabilised light source, to supply two light beams offset by a first frequency difference, a reference photodetector, connected to the outputs of the first frequency shift device and, for each photodetector, a first tuner unit connected to the output of the at least one photodetector and the output of the reference photodetector and a phase comparison unit which is connected to the output of the first tuner unit.
Because use is made of heterodyne technology, the bandwidth of the at least one photodetector must be greater than 1 kHz. Consequently, CCD detectors cannot be used and the total number of photodetectors is restricted. Furthermore, an Hexe2x80x94Ne laser is for the frequency-stabilised light source because of the good stability and the suitable wavelength.
Furthermore, in a preferred embodiment the interferometer according to the present invention is so equipped that the frequency modulation phase detection means comprise a tunable light source and a second frequency shift device, connected to the output of the tunable light source, to supply two light beams offset by a second frequency difference, the second frequency difference differing from the first frequency difference, and, for each photodetector, an associated second tuner unit, connected to the output of the at least one photodetector, and a frequency counter which is connected to the output of the second tuner unit.
The tunable light source used is preferably a semiconductor laser having an external cavity. With the aid of the external cavity it is possible to the wavelength of the light beams without changing the amplitude. The external cavity also ensures that the coherence length of the light beam is sufficiently great, so that a sufficiently large measurement range for the FM phase measuring technique is obtained. By allowing the second and first frequency difference of the second and first frequency shift device, respectively, to differ, the frequency multiplexing which has already been mentioned is made possible.
Because the interferometer also comprises a phase determination unit for each photodetector, which phase determination unit is connected to the output of the frequency counter and the output of the phase comparator, it is possible to determine the optical path difference, expressed as the phase difference between the wave fronts of the transformed object beam and the reference beam.
In one embodiment of the interferometer according to the invention, the detection means comprise an array of at least ten by at least ten photodetectors. Because the phase difference between the wave fronts of the transformed object beam and the reference beam is determined for all these photodetectors, the shape or asphericity of the reflecting body can be calculated with an adequate spatial frequency in the processing means.
In one embodiment of the interferometer according to the present invention the light source means further comprise an optical delay device for the reference beam before the latter is directed towards the optical device. As a result of the additional optical delay of the reference beam, the optical path difference between them reference beam and the object beam is reduced, as a result of which the required measurement range of the phase detection means (in particular the FM phase detection means) can be smaller.
In a further embodiment of the interferometer according to the invention the light conductors are of essentially the same length. By this means the sensitivity to mechanical and acoustic vibrations, thermal gradients and dispersion effects is reduced, which yields a more accurate and more reliable result.